Resum:

There is a long-standing interest in understanding and predicting ecosystem responses to environmental changes. In the current context of global change, the world’s ecosystems are experiencing an increase in human impact intensity leading to an unprecedented biodiversity loss. This is especially worrying in freshwater ecosystems, as they have been recognised as one of the most threatened habitats in the world. The effect of global change could be especially dramatic in semiarid areas, whose climatic conditions will become even more extreme in the short term. Thus, the loss of freshwater biodiversity is of particular concern in the Mediterranean Basin, one of the Earth’s biodiversity hotspots. Although frequently overlooked in limnological studies, riparian zones, as transitional areas between aquatic and terrestrial ecosystems, are an integral key part of freshwater ecosystems influencing both the structure and functioning of rivers. In this context, this thesis aims to explore the structural and functional responses of riparian communities to environmental filters that threaten Mediterranean river ecosystems, specially drought, land use intensification and hydromorphological alteration. It contributes to a better knowledge and prediction about the effect of natural and anthropogenic stressors on the diversity, quality and functionality of riparian and aquatic communities to improve environmental management. All the studies that compose this thesis have been carried out in the Segura River Basin. It is a Mediterranean watershed located in the southeast of the Iberian Peninsula with marked natural and anthropogenic environmental gradients. This thesis is structured in four chapters that address different aspects (diversity, ecological condition and functionality) of riparian Mediterranean communities and their relationships with environmental filters. First, in Chapter 1, the similarities and differences in taxonomic diversity patterns between woody and herbaceous riparian species are explored. Chapter 2 is focused on the effect of different types of human pressures, at different spatial scales, on the ecological condition and diversity of woody riparian plants compared with aquatic macroinvertebrate communities. The objective of Chapter 3 was to explore the functional response of riparian woodlands (using both functional diversity and redundancy measures) to environmental filters. Finally, in Chapter 4, the use of functional redundancy (it resulted as the most sensitive functional index in the previous chapter) as a tool for bioassessment is tested and compared with traditional biomonitoring indices. It can be concluded that as environmental filters intensify, riparian communities change in species composition, turning less diverse, and reducing habitat quality, functional diversity and redundancy. This jeopardises their ability to face new disturbances as well as the amount and quality of the goods and services provided by these ecosystems. Headwaters were the most favourable areas not only for riparian species but also for aquatic ones due to the absence of anthropogenic pressures and milder environmental conditions. In general, agriculture and hydromorphological alteration seemed to be the most important anthropogenic pressures for riparian and aquatic communities, respectively. In particular, the extent of agriculture upstream exerted the stronger effect on species diversity, quality and functionality of riparian communities. Accordingly, land use and hydrological planning at basin scale seem essential complements to traditional conservation and restoration measures at reach scale in order to successfully preserve rivers ecosystems. Although taxonomic richness, riparian quality and functional diversity indices seem to respond clearly to such anthropogenic impacts, functional redundancy proved to be more sensible and provided complementary information to species richness on how biological communities respond to stress. In addition, its response can be predicted from large-scale geographic variables for entire river catchments of large administrative areas. Given the clear advantages of functional redundancy over traditional biomonitoring indices, it should be incorporated into bioassessment toolbox to improve environmental management and conservation policies.